Helical potentiometer



J 28, 19 D. G. BELL 3,119,976

HELICAL' POTENTIOMETER Filed Oct. 22, 1962 2 Sheets-Sheet 2 ,q I\\ X/\\\\\ 7 flay/1w 6x24100035 INVENTOR.

United States Patent l 3,119,976 HELICAL POTENTIOMETER Donald Graham Bell, Covina, Ca-lif., assignor to Giannini Controls Corporation, Duarte, Calif., a corporation of New York Filed Oct. 22., 1962, Ser. No. 232,197 4 Claims. (Cl. 338-143) This invention relates to potentiometers, which term is employed herein in the generic sense of including rheostats.

An important object of the invention is to provide a potentiometer structure which permits movement of a sliding contact through a relatively large distance along a resistance element in response to relatively small movement of the control element.

In accordance with the invention, the resistive element of the potentiometer is arranged in a helical configuration. The movement of the control element is translational rather than rotary. In a typical arrangement, the control or input member moves axially with respect to the helical element.

However, the potentiometer structure is such that the point of contact between the wiper on the control element and the helical resistance element progresses in a rotary fashion along the length of the element in response to axial translational movement of the control member.

This novel type of movement has the great advantage that translational input movement is effectively magnified by a factor of approximately Zn'R/P, where R and P are the inner radius and the pitch, respectively, of the helix. That factor can be made quite large by selection of suitable helix proportions.

A full understanding of the invention and of its further objects and advantages will be had from the following description of certain illustrative manners in which it can be carried out. The particulars of that description and of the accompanying drawings, which form a part of it, are intended only as illustration of the invention and not as a limitation upon its scope.

In the drawings:

FIG. 1 is an axial section representing an illustrative potentiometer in accordance with the invention;

FIG. 2 is a transverse section on the line 22 of FIG. 1;

FIG. 3 is a schematic drawing in three parts illustrating contact action of a modification: Part A is in the same aspect as FIG. 1; Part B is a schematic developed view in the aspect of line B-B of Part A, and Part C is a section on line C-C of Part A;

FIG. 4 is a fragmentary section corresponding to a portion of FIG. 1 and representing a modification;

FIG. 5 is a fragmentary axial section representing a further modification;

FIG. 6 is a transverse section of FIG. 5; and

FIG. 7 is an axial section representing a further modification.

FIGS. 1 and 2 represent an illustrative embodiment of the invention, wherein the helical winding 10 with axis 11 is coaxially mounted within the cylindrical housing 26). The radially inner surface of the winding is exposed to form a contact face 12. Winding 10 typically comprises an elongated core 14 of flat section, preferably formed of slightly flexible insulative material, wound with relatively fine resistance wire. The turns of that sub-winding are preferably close together, electrical contact being prevented in the usual way by a thin coating of insulative varnish or the like on the wire. The wound core 14 is arranged in a main helix on axis 11 with sufficient helix pitch P to provide relatively large spacing between adjacent turns. The winding is typically mounted by em- 3,119,976 Patented Jan. 28, 1964 bedding its radially outer portion in a helical groove in the inner face of a sleeve of insulative material, represented at 22. That sleeve may be mounted within housing 20 in conventional manner, as by the retaining ring 23, for example. Electrical connections are indicated somewhat schematically at 16 and 17 between the respective ends of the winding and the terminals 18 and 19 on the exterior of the housing.

The wiper assembly 40 is mounted on the inner end of rod 30, and comprises the wiper carrier 42, fixedly mounted on the rod, typically in electrically insulated relation, and the wiper element 44. Wiper element 44 has a circular contact edge 48 of suitable electrically conductive material. Wiper element 44 is mounted on carrier 42 in such a way as to have limited freedom of movement parallel to a plane perpendicular to ax s 11. Such movement may be provided, for example, as in FIGS. 1 and 2 by forming a circumferential groove 46 in the outer cylindrical face of the carrier. The wiper element is of generally annular form, with its inner portion slidingly received in that groove. In the present embodiment the inner portion of the wiper element, which is received in groove 46, comprises an assembly of spring fingers Sti, typically formed of spring wire welded to a face of the wiper and extending obliquely inward in an axial plane. The fingers 50 are typically formed in pairs, as shown best in FIG. 2. They are so dimensioned as to be freely but fittingly received between the axial faces of groove 46 and to bear yieldingly on the bottom of the groove. The fingers then hold annular wiper element 44 substantially in a plane perpendicular to axis 11, and urge its movement in that plane toward a position coaxial with respect to that axis.

The radius R of the outer contact edge 4i; of the wiper is larger than the inner radius R of helical winding 10. Hence contact of the wiper edge and contact face 12 of the winding, as at 4% of FIG. 1, for example, forces the Wiper element against the force of spring fingers 50 away from coaxial relation to axis 11 by a distance b=R'-R. That displacement is sufiicient to produce a contact pressure of desired magnitude at the point of contact.

Electrical connection is provided from wiper element 44 to the external terminal 52 in any convenient manner that accommodates the wiper movement, as via the flexible insulated wire 54 and the resilient Wiper 55, for example.

Movement of the wiper of the illustrative potentiometer is controlled by means of the rod 3t which is mounted coaxially in a bore 32 in the front end wall of housing 29 for translational movement parallel to axis 11. Rod 30 may, if desired, be free to rotate, but rotary movement is not required and may be prevented by suitable means such as a spline, for example. An antifriction device of any suitable type, not specifically shown, may be provided between rod 39 and bore 32, such, for example, as an axial ball bearing.

Translational movement of rod 30 displaces contact element 44 parallel to axis 11. At its point of contact 49 with winding It the contact element slides over the winding in a generally axial direction. Such movement causes the point of contact 49 to shift circumferentially both along the contact element and along the main helix of the winding. That action changes the resistive setting of the potentiometer. The accompanying movement of the contact element relative to carrier 42 is a translational movement which carries the imaginary center point 56 of the element in a small circle 57 about main axis 11. The deflection b of the element from coaxial relation to that axis remains constant in magnitude but varies continuously in direction.

In the present embodiment, and preferably also in the embodiments to be described, the contact element is free to rotate in the plane of its contact edge about the center of that circular edge. That movement has the advantage that the contact edge receives uniform wear.

FIG. 3 represents schematically a modified form of the invention wherein the contact surface 48 of the winding has a radius of curvature of appreciable magnitude in a plane transverse of the axis 13 of the winding core. For illustration the winding is shown as a sub-helix of radius r. Despite the relatively flat form of inner winding face 12 in that embodiment, and the fact that the outer radius R of contact member 44 is greater than the inner radius R of that winding face, single point contact at 49 may be obtained provided the parts are suitably proportioned. However, FIG. 3 is not drawn to scale, but the proportions of a practical structure are exaggerated for clarity of illustration.

In FIG. 3 the line 68 represents the curve in which the plane of contact edge 43 intersects toroidal winding surface 12. A typical point of curve 68 is indicated at 72 in a plane through axis 11 at an arbitrary angle from contact point 49. The point of surface 12 in that plane closest to axis 11 is indicated at 74. In the prejection of FIG. 3A, the projected distance of point 72 from point 74 is.

d=r (l-cos (1) where is defined by FIG. 3C. From FIGS. 3B and 3C 1):? Sin =P 2 From (1) and (2), neglecting powers of 0 higher than the second,

d=P 0 /81r r (3) To the same approximation, the distance s of contact edge 48 below point 74, as seen in the projection of FIG. 3A, is

From (3) and (4):

d R P 5 4T 1- R'R (5) To obtain single point contact at 49, d must exceed s as 0 approaches zero. Since (5) is independent of 0, but is valid for small values of 6, and since R/R in practice is close to unity, the condition may be expressed as Relation (6) applies not only to a toroidal winding, but to a winding of any section provided r represents the radius of curvature of the winding contact face in a transverse plane. Condition (6) is equivalent to saying that P/ 211- exceeds the geometrical mean or r and b, where b=R'R and represents the radius of the circle 57 described by the center point 56 of contact member 44, as already described. A further alternative view of Equation 6 is that it defines a maximum value for R-R:

In practice, to obtain smooth and reliable operation it is desirable that (6) or (6a) be satisfied by an appreciable margin. It is ordinarily preferred, for example, that P/21r equal at least approximately 2 /rb.

In the above discussion, for the sake of clarity, the radius of curvature of contact edge 43 in an axial plane was assumed negligibly small compared to r, as is normally the case. If contact edge 48 is appreciably rounded, the effect of such curvature can be taken into account by suitable calculation.

FIG. 4 represents a modified structure whereby groove 46 of wiper support 42 slidingly receives and directly guides the inner edge portion of the annular contact element 44-41, Spring 50a are provided, corresponding to spring 50 of the previous embodiment, but. mounted on the inner edge of element 44a and contacting groove 46 only on its bottom surface.

FIGS. 5 and 6 represent a further illustrative manner of mounting a contact member 44b on an actuating rod, wherein the relative movement of those parts involves negligible friction. The spring supports 60 have their inner ends mounted on the rod 30a. A bearing 62 coaxial with rod 30a may be interposed between springs and rod to permit free rotation, but no mutual axial movement is permitted. The outer ends of springs 60 are secured, as by welding, to the inner face of the sleeve 64 on which contact member 44b is coaxially mounted. The springs permit radial deflection of the contact member in the manner already described, but are stifi in the axial direction and define the axial position of the contact member effectively positively.

FIG. 7 represents a further modification in which the movement of the contact element relative to its carrier involves negligible friction. The control member comprises a cylindrical sleeve 70 axially slidable in the bore 72. An elongated, slightly flexible and axially symmetrical rod '74 has one end fixedly mounted on sleeve 70. Its other end fixedly carries contact element 44c. By making the length of rod 74 large compared to the required radial deflection b=R'-R of its free end, the required radial flexibility may be obtained together with effectively positive axial definition of the contact member. Such definition is improved if the rod flexibility is effectively concentrated close to a point adjacent its fixed end, as by providing a portion 76 of reduced diameter at such point.

FIG. 7 further illustrates typical mounting of a pressure responsive bellows 80 for driving a potentiometer in accordance with the invention. Input pressure is typically supplied via the fitting indicated at 81, or the device may respond to the difference of two pressures supplied at 81 and 82. A bearing may be interposed at 84 between bellows 80 and sleeve 70 to permit free rotation of the sleeve and its carried contact member about the axis of the resistance element. Alternatively, or as a supplement to such freedom, contact member 440 can be mounted on rod 74 for free rotation about the rod axis.

I claim:

1. A potentiometer comprising in combination,

a helical resistance element having an axis and having a radially inner contact face of substantially uniform radius,

means supporting the resistance element with said face exposed,

a contact element having a rigid electrically conductive circular contact edge of fixed radius larger than said surface radius,

a carrier movable parallel to the axis of the resistance element,

means mounting the contact element on the carrier for bodily movement relative to the carrier perpendicular to the axis, the contact edge being substantially in a plane fixed with respect to the carrier and .perpendicular to the axis of the resistance element,

said mounting meansincluding means yieldingly urging the contact element toward a position coaxial with respect to the resistance element to maintain mutual electrical contact of said contact edge and contact face,

and means for making electrical connections to the respective elements,

said carrier and contact element being freely rotatable about the axis of the resistance element.

2. A potentiometer comprising in combination,

a helical resistance element having an axis and having a radially inner contact face of substantially uniform radius,

means supporting the resistance element with said face exposed,

a carrier movable parallel to the axis of the resistance element and having a coaxial groove,

an annular contact element having an electrically conductive rigid circular contact edge of fixed radius larger than said radius of the resistance element surface, and having a coaxial inner structure that is slidingly received in the carrier groove,

resilient means acting between the contact element and the carrier to yieldingly urge the contact element toward a position coaxial with respect to the carrier groove to maintain mutual electrical contact of said contact edge and contact face,

and means for making electrical connections to the respective elements.

3. A potentiometer comprising in combination,

a helical resistance element having an axis and having a radially inner contact face of substantially uniform helix pitch and inner helix radius, said face being convex in an axial plane with a substantially uniform radius of convex curvature,

means supporting the resistance element with said face exposed,

a contact element having an electrically conductive rigid circular contact flange of relatively small axial thickmess,

the outer radius of the contact flange exceeding said inner helix radius by less than the square of said helix pitch divided by 47r times said radius of convex curvature,

a carrier movable parallel to the axis of the resistance element,

means mounting the contact element for bodily movement relative to the carrier perpendicular to the axis, the contact edge being substantially in a plane perpendicular to the resistance element for relative axial and radial movement with respect thereto,

said mounting means including means yieldingly urging the contact element toward a position coaxial with respect to the resistance element to maintain mutual electrical contact of said edge and face,

and means for making electrical connections to the re spective elements.

4. A potentiometer comprising in combination,

a helical resistance element having an axis and having a radially inner contact face of substantially uniform radius,

means supporting the resistance element with said face exposed,

a contact element having a rigid electrically conductive circular contact edge of fixed radius larger than said surface radius,

a carrier movable parallel to the axis of the resistance element,

means mounting the contact element on the carrier for bodily movement relative to the carrier perpendicular to the axis, the contact edge being substantially in a plane fixed with respect to the carrier and perpendicular to the axis of the resistance element,

said mounting means including means yieldingl-y urging the contact element toward a position coaxial with respect to the resistance element to maintain mutual electrical contact of said contact edge and contact face,

and means for making electrical connections to the respective elements,

said mounting means permitting free rotation of the contact element with respect to the carrier in the plane of the contact edge about the center thereof.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A POTENTIOMETER COMPRISING IN COMBINATION, A HELICAL RESISTANCE ELEMENT HAVING AN AXIS AND HAVING A RADIALLY INNER CONTACT FACE OF SUBSTANTIALLY UNIFORM RADIUS, MEANS SUPPORTING THE RESISTANCE ELEMENT WITH SAID FACE EXPOSED, A CONTACT ELEMENT HAVING A RIGID ELECTRICALLY CONDUCTIVE CIRCULAR CONTACT EDGE OF FIXED RADIUS LARGER THAN SAID SURFACE RADIUS, A CARRIER MOVABLE PARALLEL TO THE AXIS OF THE RESISTANCE ELEMENT, MEANS MOUNTING THE CONTACT ELEMENT ON THE CARRIER FOR BODILY MOVEMENT RELATIVE TO THE CARRIER PERPENDICULAR TO THE AXIS, THE CONTACT EDGE BEING SUBSTANTIALLY IN A PLANE FIXED WITH RESPECT TO THE CARRIER AND PERPENDICULAR TO THE AXIS OF THE RESISTANCE ELEMENT, SAID MOUNTING MEANS INCLUDING MEANS YIELDINGLY URGING THE CONTACT ELEMENT TOWARD A POSITION COAXIAL WITH RESPECT TO THE RESISTANCE ELEMENT TO MAINTAIN MUTUAL ELECTRICAL CONTACT OF SAID CONTACT EDGE AND CONTACT FACE, AND MEANS FOR MAKING ELECTRICAL CONNECTIONS TO THE RESPECTIVE ELEMENTS, SAID CARRIER AND CONTACT ELEMENT BEING FREELY ROTATABLE ABOUT THE AXIS OF THE RESISTANCE ELEMENT. 